1. Field of the Invention
The present invention relates to a device for the reduction of secondary losses in the bladed flow duct of a turbomachine, wherein the device is arranged in the region of at least one of the two duct walls and, presently, intermediate two circumferentially adjacent blades, and is of the type in that the duct wall has a boundary layer step so that the face of the boundary layer step faces towards the pressure side of the blade.
2. Discussion of the Prior Art
The secondary or boundary losses in blade cascades are caused, among other factors, by the fact that the boundary layer (frictional layer proximate the wall) which enters the blade cascade and is produced along the restricting or duct walls, impelled within each individual duct by the transverse pressure gradient from the pressure side of the one blade towards the suction side of the other blade, reaches this suction side in the vicinity of the restricting or duct walls and this transverse flow (secondary flow) which is oblique relative to the base or core flow of the duct boundary layer, increases the boundary layer at the suction side (suction side boundary layer). Generated hereby are boundary or secondary eddies which extensively influence the benefits of energy conversion, particularly in axial-flow turbine blades having low aspect ratios (span-to-chord ratio of the blade).
A device of the above-mentioned description has become known through Japanese publication JP-A2 67 404.
The boundary layer step which projects forward towards the oppositely located wall of the duct, acts with its face against the oblique transverse flow. This obstacle occasions a wedge-shaped backflow of the flow or boundary layer, and upon overflowing of the edge of the boundary layer step or its face, advantageously leads to the formation of a new boundary layer on the appearance of this step. Serving for the conductance away of the fluid which is backed upon at the face is a downstream pressure dropoff gradient and/or a mixing of this fluid with the fluid of the core or base flow. No additional demands are imposed on an eventual cooling of the duct walls, and there is produced an extensively troublefree blade suction side flow near the duct walls.
The intensity of the secondary eddy displaced away from the suction side of the blade into the individual duct is lower because the velocity components resulting from the transverse pressure dropoff will not traverse the full traverse pressure dropoff gradient. The mixing losses at said step or face edge are also reduced, in essence, since the Mach-number at which the mixing takes place is overall lower than directly at the suction side of the blade.